Magnetic Particle Imaging (MPI) is an emerging medical imaging modality. The first versions of MPI were two-dimensional in that they produced two-dimensional images. Future versions will be three-dimensional (3D). A time-dependent, or 4D, image of a non-static object can be created by combining a temporal sequence of 3D images to a movie, provided the object does not significantly change during the data acquisition for a single 3D image.
MPI is a reconstructive imaging method, like Computed Tomography (CT) or Magnetic Resonance Imaging (MRI). Accordingly, an MP image of an object's volume of interest is generated in two steps. The first step, referred to as data acquisition, is performed using an MPI scanner. The MPI scanner has means to generate a static magnetic gradient field, called “selection field”, which has a single field free point (FFP) at the isocenter of the scanner. In addition, the scanner has means to generate a time-dependent, spatially nearly homogeneous magnetic field. Actually, this field is obtained by superposing a rapidly changing field with a small amplitude, called “drive field”, and a slowly varying field with a large amplitude, called “focus field”. By adding the time-dependent drive and focus fields to the static selection field, the FFP may be moved along a predetermined FFP trajectory throughout a volume of scanning surrounding the isocenter. The scanner also has an arrangement of one or more, e.g. three, receive coils and can record any voltages induced in these coils. For the data acquisition, the object to be imaged is placed in the scanner such that the object's volume of interest is enclosed by the scanner's field of view, which is a subset of the volume of scanning.
The object must contain magnetic nanoparticles; if the object is an animal or a patient, a contrast agent containing such particles is administered to the animal or patient prior to the scan. During the data acquisition, the MPI scanner steers the FFP along a deliberately chosen trajectory that traces out the volume of scanning, or at least the field of view. The magnetic nanoparticles within the object experience a changing magnetic field and respond by changing their magnetization. The changing magnetization of the nanoparticles induces a time dependent voltage in each of the receive coils. This voltage is sampled in a receiver associated with the receive coil. The samples output by the receivers are recorded and constitute the acquired data. The parameters that control the details of the data acquisition make up the scan protocol.
In the second step of the image generation, referred to as image reconstruction, the image is computed, or reconstructed, from the data acquired in the first step. The image is a discrete 3D array of data that represents a sampled approximation to the position-dependent concentration of the magnetic nanoparticles in the field of view. The reconstruction is generally performed by a computer, which executes a suitable computer program. Computer and computer program realize a reconstruction algorithm. The reconstruction algorithm is based on a mathematical model of the data acquisition. As with all reconstructive imaging methods, this model is an integral operator that acts on the acquired data; the reconstruction algorithm tries to undo, to the extent possible, the action of the model.
Such an MPI apparatus and method have the advantage that they can be used to examine arbitrary examination objects—e. g. human bodies—in a non-destructive manner and without causing any damage and with a high spatial resolution, both close to the surface and remote from the surface of the examination object. Such an arrangement and method are generally known and are first described in DE 101 51 778 A1 and in Gleich, B. and Weizenecker, J. (2005), “Tomographic imaging using the nonlinear response of magnetic particles” in nature, vol. 435, pp. 1214-1217. The arrangement and method for magnetic particle imaging (MPI) described in that publication take advantage of the non-linear magnetization curve of small magnetic particles.
The MPI technique explained above can be applied for different applications especially in human or veterinary medicine. An interesting application would also be in the field of brachytherapy or other local therapeutical applications, which the above mentioned MPI technique has not been used for so far.
Brachytheraphy is a form of radiotherapy wherein radioactive sources (often called “radioactive seeds”) are placed inside the body of a human or an animal for the treatment of, for example, prostate or cervical cancer. Within this method small radioactive seeds are implanted directly into the tumor region in order to radioactively irradiate the tumor tissue. Therefore, brachytherapy is used for internal radiation of body tissues, wherein the radiation is only limited to region of the tumor tissue itself.
Such a brachytherapy treatment method and system is, for example known from WO 2008/145377 A1. With this method the radioactive seeds are implanted into the body through hollow treatment channels, which is a common technique in state of the art brachytherapy methods. The placement of the hollow treatment channels and the amount of radiation dose to be emitted is planned before the surgery in a special treatment plan.
The state of the art brachytherapy is an invasive method which requires a complicated surgery for the implantation of the radioactive seeds. Typical problems which occur with these methods are usually the difficulty in the planning step to exactly define the position of the seeds and the amount of radiation dose to be emitted in order not to affect healthy tissue surrounding the tumor to be treated. Furthermore, it is disadvantageous with brachytherapy methods known in the art that a second surgery is required after the therapy in order to remove the implanted radioactive seeds again.
Also other medical applications are known which are similar to the above-mentioned brachytherapy method, wherein a drug or medicine has to be accurately placed at a very specific place or limited region within the body. Especially within therapeutical treatments of strokes this locally limited medication is an interesting and difficult task. According to state of the art techniques this is usually done, similar to known brachytherapy methods, by very accurately injecting or implanting the drug or medication directly to the desired position. Again, reliable non-invasive methods are so far not known for this kind of applications so that the above-mentioned methods often require complex, time-consuming and even dangerous surgical interventions within the treated object.